Many electronic circuits and components can be damaged if exposed to a high-power input signal or pulse. For example, a diode, transistor, or micro-electro-mechanical system (“MEMS”) switch can be damaged or destroyed by a high-power radio-frequency (“RF”) signal, an electro-static discharge (“ESD”) event, or in general any sufficiently large form of electrical overstress. Various techniques are used to avoid such damage.
Passive limiters are used in a number of circuits, such as at the input to microwave and RF instrumentation and components, to limit the amount of power at the output of the passive limiter. Passive limiters are typically diode-based and are designed to operate so that the limiter starts attenuating at a selected RF input signal level, and increases attenuation as the RF input signal level increases. The components in a passive limiter, which are frequently diodes, transition from one conductive state to another according to the RF signal power level, hence the term “passive” limiter. Generally, the power level at the output of the passive limiter does not rise above a maximum value, regardless of the power at the input to the limiter, as long as the input power does not exceed the power-handling ability of the passive limiter.
However, a disadvantage of passive limiters is that a tradeoff is made between the power level at which the passive limiter starts attenuating an input signal and the amount of distortion that a passive limiter adds to the input signal. For example, in order to avoid damaging a low-noise amplifier that may be selectively switched into the input path of a microwave test instrument, a maximum power of one watt is desired. However, because the same microwave test instrument makes measurements up to a power level of one watt (e.g. when the low-noise amplifier is not in the input signal path), the threshold power level of the passive attenuator is set to two watts or more to avoid the distortion that would otherwise occur on one-watt input signals if the threshold power level was set lower. Even with the threshold power level set to two watts, there can be significant distortion added to a one-watt input signal because of the “soft” way the limiter turns on. The tradeoff between power limiting threshold and distortion is becoming more important with more complex modulation formats because complex modulation formats are more susceptible to distortion caused by passive limiters, particularly at high signal power levels.
Some limiters have their limiting threshold (i.e. the threshold power level at which the limiter starts attenuating) set by an external voltage. Examples of such limiters have been built as integrated circuits (“ICs”). An IC limiter may be used in a variety of applications by selecting the appropriate external applied voltage to provide the desired limiting threshold. Unfortunately, this style of limiter still adds significant distortion when used in a frequency domain application at high power levels.
Another technique that has been used is active limiting. Active power limiting is the process of actuating a limiter circuit by an external control voltage or current according to a detected input power level. Active power limiters have been built using distributed components (e.g. limiter ICs and diodes) on a printed circuit board (“PCB”) or other substrate. In a particular example, current draw through the diodes of one section of a distributed limiter is monitored and used to infer RF overpower conditions. Information gained by sensing the current is used to control a switch that opens a mechanical relay, thus preventing damage to downstream components.
The active limiter uses a passive diode GaAs IC limiter followed by a number of passive diodes in surface mount technology (“SMT”) packages. The SMT diodes and GaAs IC limiter are mounted on a PCB to both sense and limit RF overpower conditions and ESD pulses, as well as to control a mechanical relay. These components protect downstream circuits from power up to 50 watts.
However, this active limiter is physically large due to the number of distributed components. This active limiter has a maximum frequency of operation around 6 GHz, due mainly to the mechanical relay. Furthermore, this active limiter will only reliably limit high-power signals a limited number of times before contacts of the mechanical relay are degraded.
Therefore, an active limiter that operates at higher frequencies, is physically smaller than conventional active limiters, and can be set to its power limiting state more quickly is desired.